1. Field of the Invention
The present invention relates to an integrated circuit device such as a heat-sensitive recording head.
2. Description of the Prior Art
The heat-sensitive recording head (which will be shortly referred to as a "head") is constructed such that heating portions abutting directly or through an ink ribbon against a recording medium such as heat-sensitive paper or recording paper are heated selectively in a dot shape by recording electric signals so that an image can be recorded on the recording medium.
In the head thus constructed, generally speaking: a heating layer is formed on an insulating substrate; a multiplicity of opposed electrodes are formed on the heating layer to form a heating portion; this heating portion is covered with an insulating cover film to protect the opposed electrodes and the heating layer; and signal electrodes constructing those opposed electrodes are fed with, as in the later-described line type, for example, with sigaals corresponding to an objective image pattern from an integrated circuit (which will be shortly referred to as an "IC").
In the head of the later-described serial type, for example, as shown in FIG. 14 and FIG. 15 providing a section taken along line XV--XV of FIG. 14, a glazing layer 216 formed over an insulating substrate 203 is covered with a heating layer 208, which is covered with opposed electrodes composed of a common electrode 209 and individual signal electrodes 210 facing each other through a gap 211 forming a heating portion 202. The heating layer 208 and the electrodes 209 and 210 underlying the electrode gap 211 are covered with and protected by a protecting layer 217. Incidentally, this protecting layer 217 ss omitted from FIG. 14.
In this head, the heating portion 202 abutting directly or through an ink ribbon against a recording medium such as heat-sensitive paper or recording paper is heated selectively in a dot shape by recording electric signals so that an image can be recorded on the recording medium.
Since the aforementioned electric signals are delivered through the signal electrodes 210, the density of the current flowing through the common electrode 209 increases, when the number of the heating portions to be selectively heated increases, so that the common electrode is broken by the electro-migration. In order to obviate this, the width of flat portion of the common electrode 209 has to be as large as about 0.5 to 2.0 mm with respect to that of the signal electrodes 210 of 50 to 200 .mu.m.
In Europe and America, on the other hand, rough paper of smoothness of 5 to 10 sec having a coarse surface is preferred as the recording paper. This rough paper has to be forced into contact with the heating portions 202 by a strong pressure because the record (or print) is made very unclear by the ordinary recording method. As the means for applying the strong pressure, as indicated by a phantom line in FIG. 15, a recording paper 30 is preferably forced to contact at an inclination with the head. In the head of the prior art, however, the recording paper 30 cannot be inclined with respect to the head because it is obstructed by the wide common electrode 209 arranged at the side edge of the head. This arrangement of the wide common electrode 209 at the head side edge obstructs reduction of the size of the head, too, in another integrated circuit device.
Especially in the head, it is the recent trends tha the dot number is increased so as to improve the quality of the printed or recorded image and that the size of the substrate is reduced so as to drop the production cost. With the dot number increased, the common electrode has to be widened so as to obviate the electro-migration, and the substrate itself is enlarged. In order to meet the requirement that the recorded result is observed instantly after the recording, it is necessary to position the heating portions as close to the en face of the head as possible. This necessity is not substantially satisfied because the wide common electrode is arranged at the side edge of the substrate.
I , the Inventor, have found, as a result of our keen investigations, that the aforementioned problems are solved by forming the common electrode over the glazing layer, by sequentially laminating the insulating layer, the heating layer and the signal electrodes on the common electrode, while leaving a portion of the same, to form the heating portion, and by preventing the wide common electrode from occupying the side edge of the substrate. As a result of further investigations, I also have found, in the head thus constructed, that the temperature of the common electrode under the heating portions is raised to a high one if the dot density (i.e., the density of the heating portions) is further increased so as to obtain a recorded result of higher resolution, and is held at 200.degree. to 300.degree. C. even when no power is applied. When the common electrode becomes hot, the alkaline component such as Na contained in the glazing layer is ionized to diffuse through the common electrode into the heating layer to cause the aging of the resistance of the heating layer thereby to shorten the lifetime of the head. Incidentally, the head lifetime is assumed to terminate at the instant when the resistance changes .+-.15% with respect to a set value. In the structure thus far described, moreover, the material for the common electrode is a metal of high melting point because the insulating layer is formed at a high temperature. It is also found that the main component SiO.sub.2 of the glazing layer and the material of the common electrode are subjected to a solid state reaction, when the common electrode becomes hot, to oxidize the common electrode and degrade the adhesiveness of the same.